There is a petition you can fill out online asking the Attorney General to write an opinion as to weather a 5320.1 or 5320.4 constitute a "license" to own regulated items like suppressors in accordance to state law.

In 2005, the Attorney General wrote an opinion that the 5420.4 was indeed a de-facto license for the purposes of owning a machine gun in Michigan. The petion asks that they make a ruling regarding suppressors as well. LINK HERE.

SBR's and SBS's are outlawed in Michigan as a matter of law, but suppressors are legal, as long as you are licesned. The problem is, there is no such "license" for ownership available from the Federal Government, nor the State Government.